The invention relates to fabricating thermostructural composite material parts, in particular parts for use in the field of aviation or in the field of space. Examples of such parts are afterbody elements for gas turbine aeroengines, such as mixers or secondary nozzles of multiple-flow engines or central bodies or “plugs”.
Thermostructural composite materials are remarkable for their mechanical properties, which make them suitable for constituting structural parts, and for their ability to conserve these properties at high temperatures. Well-known thermostructural composite materials are carbon/carbon (C/C) composites, comprising carbon fiber reinforcement with a carbon matrix, and ceramic matrix composites (CMC) comprising refractory fiber reinforcement (carbon fiber or ceramic fiber) and a ceramic matrix. CMCs present not only very good mechanical strength at high temperature, but also good ability to withstand a corrosive environment (the presence of oxidizers and moisture). Use of CMCs has therefore already been proposed for the afterbody elements of aeroengines that are subjected in operation to temperatures generally lying in the range 400° C. to 750° C.
One known process for fabricating a thermostructural composite material part comprises the following steps:                making a fiber preform out of yarns (or tows) of carbon or of ceramic impregnated by a consolidating composition containing a carbon- or ceramic-precursor, generally a resin that is optionally diluted in a solvent;        transforming the carbon- or ceramic-precursor by pyrolysis; and then        densifying the preform by chemical vapor infiltration (CVI).        
In order to make the impregnated fiber preform, one or more plies of fiber texture are used, e.g. a three-dimensional (3D) woven fabric, impregnated with the consolidating composition, and the fiber texture is shaped, e.g. by being draped on conformation tooling, so as to obtain a preform having a shape that corresponds to the shape of the part to be fabricated. The resin of the consolidating composition is cured and then pyrolyzed so as to leave a solid carbon or ceramic residue that serves to consolidate the preform. The consolidated preform is densified with a carbon or ceramic matrix obtained by CVI. In well-known manner, CVI densification is performed by placing the consolidated preform in a reaction chamber and by introducing a reaction gas into the chamber, the reaction gas containing one or more carbon- or ceramic-precursors, with conditions in the reaction chamber, in particular pressure and temperature conditions, being selected so as to enable the reaction gas to diffuse within the pores of the preform and form therein a solid deposit of carbon or ceramic by decomposition of one or more of the components of the reaction gas or by reaction between a plurality of its components.
The impregnation with the consolidating composition needs to be performed so as to be sufficient to obtain the quantity of solid residue after pyrolysis that is necessary for satisfactory consolidation. The term “satisfactory consolidation” is used herein to mean partial densification of the fiber preform that reaches or slightly exceeds a threshold beyond which the preform on its own conserves its shape and may be handled, if necessary, without requiring tooling to hold it. The Applicant has observed that sufficient consolidation is generally obtained with the solid residue after pyrolysis has a volume percentage of 12% to 14% (i.e. the percentage of the apparent volume of the preform that is occupied by the solid residue).
Mechanical tests performed on CMC parts obtained in this way by consolidation using a liquid technique with a ceramic precursor resin and densification by CVI have given results that are satisfactory from a thermomechanical point of view, but for which it may be desirable to obtain improvements concerning the Young's modulus of the material. The Applicant has observed that because of the quantity of consolidating composition that is required, a large portion of the intra-yarn spaces is occupied by the solid residue of pyrolysis, thereby imparting mechanical properties that are not as good as those provided by the ceramic matrix obtained by CVI.